Controlling the efficiency of spin injection into graphene by carrier drift

Electrical spin injection from ferromagnetic metals into graphene is hindered by the impedance mismatch between the two materials. This problem can be reduced by the introduction of a thin tunnel barrier at the interface. We present room temperature non-local spin valve measurements in cobalt/aluminum-oxide/graphene structures with an injection efficiency as high as 25%, where electrical contact is achieved through relatively transparent pinholes in the oxide. This value is further enhanced to 43% by applying a DC current bias on the injector electrodes, that causes carrier drift away from the contact. A reverse bias reduces the AC spin valve signal to zero or negative values. We introduce a model that quantitatively predicts the behavior of the spin accumulation in the graphene under such circumstances, showing a good agreement with our measurements.Comment: 4 pages, 3 color figure

Field and numerical study of river confluence flow structures

River hydrodynamicsTurbulent open channel flow and transport phenomen

Linear scaling between momentum and spin scattering in graphene

Spin transport in graphene carries the potential of a long spin diffusion length at room temperature. However, extrinsic relaxation processes limit the current experimental values to 1-2 um. We present Hanle spin precession measurements in gated lateral spin valve devices in the low to high (up to 10^13 cm^-2) carrier density range of graphene. A linear scaling between the spin diffusion length and the diffusion coefficient is observed. We measure nearly identical spin- and charge diffusion coefficients indicating that electron-electron interactions are relatively weak and transport is limited by impurity potential scattering. When extrapolated to the maximum carrier mobilities of 2x10^5 cm^2/Vs, our results predict that a considerable increase in the spin diffusion length should be possible

Electronic spin transport in graphene field effect transistors

Spin transport experiments in graphene, a single layer of carbon atoms, indicate spin relaxation times that are significantly shorter than the theoretical predictions. We investigate experimentally whether these short spin relaxation times are due to extrinsic factors, such as spin relaxation caused by low impedance contacts, enhanced spin flip processes at the device edges or the presence of an aluminium oxide layer on top of graphene in some samples. Lateral spin valve devices using a field effect transistor geometry allowed for the investigation of the spin relaxation as a function of the charge density, going continuously from metallic hole to electron conduction (charge densities of $n\sim 10^{12}$cm$^{-2}$) via the Dirac charge neutrality point ($n \sim 0$). The results are quantitatively described by a one dimensional spin diffusion model where the spin relaxation via the contacts is taken into account. Spin valve experiments for various injector/detector separations and spin precession experiments reveal that the longitudinal (T$_1$) and the transversal (T$_2$) relaxation times are similar. The anisotropy of the spin relaxation times $\tau_\parallel$ and $\tau_\perp$, when the spins are injected parallel or perpendicular to the graphene plane, indicates that the effective spin orbit fields do not lie exclusively in the two dimensional graphene plane. Furthermore, the proportionality between the spin relaxation time and the momentum relaxation time indicates that the spin relaxation mechanism is of the Elliott-Yafet type. For carrier mobilities of 2-5$\times 10^3$ cm2^/Vs and for graphene flakes of 0.1-2 $\mu$m in width, we found spin relaxation times of the order of 50-200 ps, times which appear not to be determined by the extrinsic factors mentioned above.Comment: 11 pages, 13 figure

A GBT Survey of the HALOGAS Galaxies and Their Environments I: Revealing the full extent of HI around NGC891, NGC925, NGC4414 & NGC4565

We present initial results from a deep neutral hydrogen (HI) survey of the HALOGAS galaxy sample, which includes the spiral galaxies NGC891, NGC925, NGC4414, and NGC4565, performed with the Robert C. Byrd Green Bank Telescope (GBT). The resulting observations cover at least four deg$^2$ around these galaxies with an average 5$\sigma$ detection limit of 1.2$\times$10$^{18}$ cm$^{-2}$ over a velocity range of 20 km s$^{-1}$ and angular scale of 9.1$'$. In addition to detecting the same total flux as the GBT data, the spatial distribution of the GBT and original Westerbork Synthesis Radio Telescope (WSRT) data match well at equal spatial resolutions. The HI mass fraction below HI column densities of 10$^{19}$ cm$^{-2}$ is, on average, 2\%. We discuss the possible origins of low column density HI of nearby spiral galaxies. The absence of a considerable amount of newly detected HI by the GBT indicates these galaxies do not have significant extended diffuse HI structures, and suggests future surveys planned with the SKA and its precursors must go \textit{at least} as deep as 10$^{17}$ cm$^{-2}$ in column density to significantly increase the probability of detecting HI associated with the cosmic web and/or cold mode accretion.Comment: Accepted for publication in The Astrophysical Journal; 28 pages, 15 figure

HALOGAS observations of NGC 5023 and UGC 2082: Modeling of non-cylindrically symmetric gas distributions in edge-on galaxies

In recent years it has become clear that the vertical structure of disk galaxies is a key ingredient for understanding galaxy evolution. In particular, the presence and structure of extra-planar gas has been a focus of research. The Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey aims to provide a census on the rate of cold neutral gas accretion in nearby galaxies as well as a statistically significant set of galaxies that can be investigated for their extra-planar gas properties. In order to better understand the the vertical structure of the neutral hydrogen in the two edge-on HALOGAS galaxies NGC 5023 and UGC 2082 we construct detailed tilted ring models. The addition of distortions resembling arcs or spiral arms significantly improves the fit of the models to these galaxies. In the case of UGC 2082 no vertical gradient in rotational velocity is required in either symmetric models nor non-symmetric models to match the observations. The best fitting model features two arcs of large vertical extent that may be due to accretion. In the case of NGC 5023 a vertical gradient is required in symmetric models (dV/dz =$-14.9\pm3.8$ km s$^{-1}$ kpc$^{-1}$) and its magnitude is significantly lowered when non-symmetric models are considered (dV/dz =$-9.4\pm3.8$ km s$^{-1}$ kpc$^{-1}$). Additionally it is shown that the underlying disk of NGC 5023 can be made symmetric, in all parameters except the warp, in non-symmetric models. In comparison to the "classical" modeling these models fit the data significantly better with a limited addition of free parameters.Comment: 27 Pages, 22 Figures. Accepted for publication in MNRA

Effect of tributary inflow on reservoir turbidity current

Abstract: Fluvial flows carrying high sediment loads may plunge into reservoirs to form turbidity currents. However, the effects of tributary inflows on reservoir turbidity currents have remained poorly understood to date. Here a 2D double layer-averaged model is used to investigate a series of laboratory-scale numerical cases. By probing into the hydro-sediment-morphodynamic processes, we find that tributary location and inflow conditions have distinct effects on the formation and propagation of reservoir turbidity currents, and lead to complicated flow dynamics and bed deformation at the confluence. Two flow exchange patterns are generated at the confluence: turbidity current intrusion from the main channel into the tributary; and highly concentrated, sediment-laden flow plunging from the tributary into the turbidity current in the main channel. Tributary sediment-laden inflow may cause the stable plunge point to migrate downstream and is conducive to propagation of the turbidity current, whilst the opposite holds in the case of clear-water inflow from the tributary. Tributary inflow leads to a lower sediment flushing efficiency as compared to its counterpart without a tributary. Yet a high sediment concentration in the tributary may reinforce turbidity current in the reservoir, thereby increasing sediment flushing efficiency. Around the confluence, the planar distributions of velocity and bed shear stress of the turbidity current resemble their counterparts in confluence flows carrying low sediment loads or clear water. Yet, the bed exhibits aggradation near the confluence due to the turbidity current, in contrast to pure scour in a river confluence with a low sediment load. Appropriate account of tributary effects is required in studies of reservoir turbidity currents, and for devising strategies for long-term maintenance of reservoir capacity. Article highlights: Tributary inflow may cause the stable plunge point of reservoir turbidity current to migrate either upstream or downstream and modify its propagation.Tributary inflow may lead to lower sediment flushing efficiency by reservoir turbidity current.Tributary discharge and sediment concentration may lead to disparate bed deformation at confluence